A. Field
This invention relates to an improved membrane separation device of the spiral wound type useful for ultrafiltration, microfiltration and reverse osmosis applications and capable of obtaining high conversions while maintaining turbulent or chopped laminar hydrodynamic flow conditions, including methods of use. More specifically, the invention relates to a spiral wound membrane element device having a radial feed path ("RFP" herein) and thereby providing a potential for much higher conversion rates in a single element than heretofore possible.
B. Description of the Prior Art
Spiral membrane elements for ultrafiltration, microfiltration and reverse osmosis have long been regarded as efficient devices for separating components of fluid mixtures. In a typical process, a pressurized fluid mixture is brought into contact with a membrane surface whereby one or more components of that fluid mixture pass through the membrane because of a difference in chemical potential and, due to varying mass transport rates through the membrane, a separation is achieved.
The most common spiral membrane element known heretofore is designed to have the fluid feed mixture enter at one end of the cylindrical membrane element and travel across the spiral windings between parallel membrane surfaces along the longitudinal axis of the element (axial feed path-"AFP" herein). Separation occurs at the membrane-fluid interface resulting in (1) a more concentrated feed stream and (2) a permeate, which is the fluid passing through the membrane barrier layer. The permeate stream travels in a spiraling radial direction within the separate sealed channel defined by the permeate sides of two membranes until it reaches the porous central core tube where it is collected and expelled out one or both ends of the core tube (see, U.S. Pat. Nos. 4,235,723, 3,367,504, 3,504,796, 3,493,496, 3,417,870).
Spiral wound membranes invariably contain a flow path or channel for the feed enclosed by membrane sheets with active membrane barrier layers facing said flow path. In the case of anisotropic membranes containing a single barrier layer on only one side of the sheet, it is conventional for the membrane sheets to have the barrier layers facing each other and separated by a spacer which promotes turbulence in the feed flow path. The membranes are edge-sealed with adhesives or heat sealed, etc. in such a manner as to furnish an inlet for feed and an outlet for concentrate (since "feed" becomes "concentrate" as it passes along the membrane, the stream within the membrane element may be optionally termed "feed-concentrate" herein).
The conversion (i.e. the ratio of permeate volume to feed volume) for the common prior art spiral elements is governed by the element's length (see, Desalination by Reverse Osmosis, Ulrich Merten, 1966, Chapter 5). Typically, unit conversions are far below commercial process requirements, requiring numerous elements in series to achieve acceptable converions. For example, a typical remote osmosis system operating at 75% conversion might require eighteen one meter long elements in a 2-1 array of pressure vessels producing a feed-concentrate flow path length of 12 meters (i.e., first stage has six elements in series in each of two parallel trains and the second stage has six elements in series in a single train). The requirement for arraying spiral elements in series depending on the fouling potential of the feed water with the above example being most commonly employed on municipal, well, and surface-water feeds without extraordinary pretreatment.
For desalination systems requiring high conversions and permeate flows below 75,000 to 100,000 GPD, (gallons per day), small diameter elements (less than 8 inches) must be used to maintain arrays with 12 meter feed-concentrate path lengths. The drawbacks to this method of obtaining high conversion include (a) increased floor space requirements, (b) increased membrane module cost on a cents per gallon basis, (c) increased process and pressure vessel costs, and (d) added complexity of expanding systems due to array requirements.
If it were possible to change the element flow path from the standard axial (AFP) to a radial direction (RFP), the flow path may be tailored to the desired conversion rate or even increased; thus such module's conversion would be governed by its diameter rather than length.
Unfortunately, it is not a simple matter to design a practical radial flow path element since the permeate collected within the permeate channel must not travel more than one to two meters before exiting the module, or excess back pressure is generated in the permeate carrier fabric reducing the element's efficiency. This constraint eliminates the possibility of successfully utilizing the principle of the flowpath design of U.S. Pat. No. 3,933,646 containing one or more very long membrane envelopes in which the permeate travels the length of the membrane envelope before entering the core tube and exiting the module.